1. Field of the Invention
The present invention relates to high-strength heat-resistant steels, and particularly to high-strength heat-resistant steels which are suitable for use in a medium-to-high temperature range up to 540.degree. C., and which can be produced at a low cost.
This application is based on Patent Applications Nos. Hei 10-272202 and Hei 11-40618, both filed in Japan, the contents of which are incorporated herein by reference.
2. Description of Related Art
Large portions of materials for pressure-tight parts of piping for use in the highest temperature sections of subcritical-pressure boilers and supercritical-pressure boilers in power plants and waste heat recovery boilers in combined cycle power plants, and semi-high temperature sections of ultra supercritical-pressure boilers are carbon steels and low alloy steels such as 1 Cr steel and 2 Cr steel.
Specific examples of low alloy steels which have been used are 0.5 Mo steel (JIS STBA 12), 1 Cr--0.5 Mo steel (JIS KA STBA 21, STBA 22, STBA 23) and 2.25 Cr--1 Mo steel (JIS STBA 24).
Since large portions of the materials for pressure-tight parts of piping are carbon steels and low alloy steels such as 1Cr steel and 2Cr steel, achievement of sufficient strength of the materials for the parts in which they are used, without increasing the use of alloying elements, would largely contribute to reducing the cost for constructing a power plant.
In Japanese Unexamined Patent Application, First Publication (Kokai) No. Hei 10-195593, the present inventors proposed a steel excellent in high-temperature strength as a material suitable for the above uses, comprising C in an amount of 0.01 to 0.1% by weight, Si in an amount of 0.15 to 0.5% by weight, Mn in an amount of 0.4 to 2% by weight, V in an amount of 0.01 to 0.3% by weight, and Nb in an amount of 0.01 to 0.1% by weight, the balance being Fe and unavoidable impurities.
The heat-resistant steel proposed as above is a useful steel, which possesses an enhanced high-temperature strength in comparison with conventional steels although it can be produced at a low cost. However, further enhancement of the high-temperature strength is desired without increasing the cost.